1. Field of the Invention
This invention relates to a magnetic recording medium which can maintain high electromagnetic transfer characteristics and quality reliability even when coating layers including a magnetic layer are made thinner or a total thickness of a tape is reduced, and a process for preparing the same.
2. Prior Art
In recent years, higher demands have increasingly been made for a high-recording density magnetic recording medium. In order to respond to these demands, it has been well known for those skilled in the art that recording density can be improved by using ferromagnetic metallic powder as magnetic powder. However, it is insufficient to fulfill the demands made in recent years for the high-recording density magnetic recording medium only by such a method.
Thus, as other method for responding to a demand to improve a reproduction output with a wide range from a high frequency to a low frequency, it has been carried out to make the magnetic recording medium a multilayer structure in which an upper layer comprises ferromagnetic metallic powder corresponding to a high frequency and a lower layer comprises iron oxide magnetic powder corresponding to a low frequency.
Further, for a demand to establish higher recording density, there is a method of decreasing self-demagnetization and reproduction demagnetization by making the magnetic layer thinner. This is also the method of making a magnetic recording medium a multilayer structure in which an upper layer is a thin magnetic layer and a lower layer is a non-magnetic layer. In addition to these methods, it has been proposed to improve recording density by improving ferromagnetic metallic powder.
However, the demands for the high-recording density magnetic recording medium include not only improvement of electromagnetic transfer characteristics as described above, but also a thinner magnetic recording medium for miniaturization and use for a long period of time. A demand to make a support thinner is also included therein, and in order to maintain strength of a support, it has been investigated to use polyethylene naphthalate (PEN), or aramid having higher strength as a material for a support in place of polyethylene terephthalate (PET). However, a support having high strength is expensive, which causes an increase in cost of a medium. If it is possible to make coating layers thinner, a support having a certain thickness can be used, it is not necessary to use a material having high strength and a coating amount can be reduced, whereby a significant decrease in cost can be realized.
However, in conventional methods, when a coating layer is made thinner, it is difficult to smooth a surface by smoothing treatment of a magnetic layer surface (calendering) under the same conditions. As the coating layer is thinner, surface roughness is increased significantly and electromagnetic transfer characteristics are lowered extremely, whereby a resulting medium cannot be put into practical use. Also, there has been attempted a method of smoothing a surface of a thin coating layer by strengthening calendering conditions, but it has been found that when the surface is smoothed by this method, durability of a magnetic recording medium is lowered extremely.
As described above, when a coating layer is made thinner, both of high electromagnetic transfer characteristics and durability cannot be obtained at the same time by conventional techniques.